A gravel pack is put in place by a procedure using a carrier liquid in which the particulate material is suspended. The carrier liquid is usually aqueous. Typically the carrier liquid with suspended material in it is pumped into the space around a sand screen. The carrier liquid flows into the production tube through apertures provided for that purpose and returns towards the surface but particulate material cannot pass through a sand screen or small apertures giving access to the production tube and is retained as a pack in the wellbore outside the production tube. Some of the carrier liquid may leak off into the formation around the wellbore, either through perforations in the casing or directly into the formation if the bore is an open hole without casing. A gravel pack may be put in place as part of the operation of completing a wellbore for production.
Although a gravel pack is envisaged primarily as a component of a wellbore with the pack located within the drilled borehole, it is possible for a gravel pack to extend somewhat into the formation around the wellbore. This will happen when a gravel pack is put in place after perforation of the wellbore casing. In this event the gravel pack will occupy an annulus of the wellbore around the production tube and may also extend into the perforations through the casing and into the formation.
A gravel pack also extends into the surrounding formation when the so-called “frac and pack” process is used. In this process a fracturing treatment is carried out, generally on a small scale so as to produce small fractures, followed by placing a gravel pack which extends into the fractures as well as occupying the annulus around the production tube. Literature discussion of the frac and pack process can be found for instance in PETSOC 2001-055 which is a paper of the Petroleum Society of the Canadian Institute of Mining, Metallurgy & Petroleum. In this frac and pack process the pack is transported into place using a carrier liquid. Some of the liquid carries particulate material into the fractures and then leaks off into the formation while some of the liquid enters the production tube and returns towards the surface.
A variety of forms of apparatus may be utilized for delivering the carrier liquid and suspended solid to the location where the gravel pack is required and a variety of carrier liquids may also be employed, but the suspended particulate material generally has a specific gravity which is higher than that of the carrier liquid and so the particulate material will tend to settle out. This settling out of the particulate material can cause, or exacerbate, blockages and/or incomplete packing of the volume which it is desired to fill with the particulate material.
There have been two approaches to the formulation of the carrier liquid. One approach includes a viscosity-enhancing thickening agent in the liquid, which retards settling of solid materials. Guar and other polysaccharides have been widely used for this purpose. Chemically modified polysaccharides and viscoelastic surfactants have also been used. An alternative is to use a low viscosity carrier liquid. This considerably reduces the energy required in pumping but keeping particulate material in suspension becomes much more difficult and a higher pump flow rate is commonly used so that shear and turbulence keep the particulate solid in suspension for transport. Society of Petroleum Engineers paper SPE38640 mentions that both types of fluid are in use and references a number of papers discussing the relative strengths and weaknesses of the different fluids.
Sand, which has a specific gravity of 2.65, is widely used as the particulate material for gravel packs. Material of lower specific gravity has been used and is mentioned in SPE 38640. Subsequent papers, including SPE98298 and SPE106364 describe the placing of gravel packs in horizontal wellbores using lightweight materials which are also known for use as lightweight proppants in fractures. The lightweight material referred to in these papers is based on comminuted walnut hulls to which a resin coating has been applied. It has a specific gravity of 1.25. Lightweight particulate materials tend to be associated with lower strength than sand. In the context of a gravel pack this may not be an issue but strength is required in a frac and pack procedure where some material must act as proppant for the fractures.